Rules of Mixture for Elastic Properties

'Rules of Mixtures' are mathematical expressions which give some property of the composite in terms of the properties, quantity and arrangement of its constituents. They may be based on a number of simplifying assumptions, and their use in design should tempered with extreme caution!

Density For a general composite, total volume V, containing masses of constituents Ma, Mb, Mc,... the composite density is In terms of the densities and volumes of the constituents:

Density But va / V = Va is the volume fraction of the constituent a, hence: For the special case of a fibre-reinforced matrix: since Vf + Vm = 1

Rule of mixtures density for glass/epoxy composites rf rm

Micromechanical models for stiffness

Unidirectional ply Unidirectional fibres are the simplest arrangement of fibres to analyse. They provide maximum properties in the fibre direction, but minimum properties in the transverse direction. fibre direction transverse direction

Unidirectional ply We expect the unidirectional composite to have different tensile moduli in different directions. These properties may be labelled in several different ways: E1, E|| E2, E

Unidirectional ply By convention, the principal axes of the ply are labelled ‘1, 2, 3’. This is used to denote the fact that ply may be aligned differently from the cartesian axes x, y, z. 3 1 2

Unidirectional ply - longitudinal tensile modulus We make the following assumptions in developing a rule of mixtures: Fibres are uniform, parallel and continuous. Perfect bonding between fibre and matrix. Longitudinal load produces equal strain in fibre and matrix.

Unidirectional ply - longitudinal tensile modulus A load applied in the fibre direction is shared between fibre and matrix: F1 = Ff + Fm The stresses depend on the cross-sectional areas of fibre and matrix: s1A = sfAf + smAm where A (= Af + Am) is the total cross-sectional area of the ply

Unidirectional ply - longitudinal tensile modulus Applying Hooke’s law: E1e1 A = Efef Af + Emem Am where Poisson contraction has been ignored But the strain in fibre, matrix and composite are the same, so e1 = ef = em, and: E1 A = Ef Af + Em Am

Unidirectional ply - longitudinal tensile modulus Dividing through by area A: E1 = Ef (Af / A) + Em (Am / A) But for the unidirectional ply, (Af / A) and (Am / A) are the same as volume fractions Vf and Vm = 1-Vf. Hence: E1 = Ef Vf + Em (1-Vf)

Unidirectional ply - longitudinal tensile modulus E1 = Ef Vf + Em ( 1-Vf ) Note the similarity to the rules of mixture expression for density. In polymer composites, Ef >> Em, so E1  Ef Vf

This rule of mixtures is a good fit to experimental data (source: Hull, Introduction to Composite Materials, CUP)

Unidirectional ply - transverse tensile modulus For the transverse stiffness, a load is applied at right angles to the fibres. The model is very much simplified, and the fibres are lumped together: L2 matrix fibre Lf Lm

Unidirectional ply - transverse tensile modulus It is assumed that the stress is the same in each component (s2 = sf = sm). Poisson contraction effects are ignored.

Unidirectional ply - transverse tensile modulus Lf Lm The total extension is d2 = df + dm, so the strain is given by: e2L2 = efLf + emLm so that e2 = ef (Lf / L2) + em (Lm / L2)

Unidirectional ply - transverse tensile modulus Lf Lm But Lf / L2 = Vf and Lm / L2 = Vm = 1-Vf So e2 = ef Vf + em (1-Vf) and s2 / E2 = sf Vf / Ef + sm (1-Vf) / Em

Unidirectional ply - transverse tensile modulus Lf Lm But s2 = sf = sm, so that: or

Note that E2 is not particularly sensitive to Vf. If Ef >> Em, E2  Em / (1-Vf) Note that E2 is not particularly sensitive to Vf. If Ef >> Em, E2 is almost independent of fibre property:

carbon/epoxy glass/epoxy The transverse modulus is dominated by the matrix, and is virtually independent of the reinforcement.

The transverse rule of mixtures is not particularly accurate, due to the simplifications made - Poisson effects are not negligible, and the strain distribution is not uniform: (source: Hull, Introduction to Composite Materials, CUP)

Unidirectional ply - transverse tensile modulus Many theoretical studies have been undertaken to develop better micromechanical models (eg the semi-empirical Halpin-Tsai equations). A simple improvement for transverse modulus is where

Generalised rule of mixtures for tensile modulus E = hL ho Ef Vf + Em (1-Vf ) hL is a length correction factor. Typically, hL  1 for fibres longer than about 10 mm. ho corrects for non-unidirectional reinforcement:

Theoretical Orientation Correction Factor ho = S ai cos4 qi Where the summation is carried out over all the different orientations present in the reinforcement. ai is the proportion of all fibres with orientation qi. E.g. in a ±45o bias fabric, ho = 0.5 cos4 (45o) + 0.5 cos4 (-45o)

Assuming that the fibre path in a plain woven fabric is sinusoidal, a further correction factor can be derived for non-straight fibres:

Theoretical length correction factor Theoretical length correction factor for glass fibre/epoxy, assuming inter-fibre separation of 20 D.

Stiffness of short fibre composites For aligned short fibre composites (difficult to achieve in polymers!), the rule of mixtures for modulus in the fibre direction is: The length correction factor (hL) can be derived theoretically. Provided L > 1 mm, hL > 0.9 For composites in which fibres are not perfectly aligned the full rule of mixtures expression is used, incorporating both hL and ho.

In short fibre-reinforced thermosetting polymer composites, it is reasonable to assume that the fibres are always well above their critical length, and that the elastic properties are determined primarily by orientation effects. The following equations give reasonably accurate estimates for the isotropic in-plane elastic constants: where E1 and E2 are the ‘UD’ values calculated earlier

Rules of mixture properties for CSM-polyester laminates Larsson & Eliasson, Principles of Yacht Design

Rules of mixture properties for glass woven roving-polyester laminates Larsson & Eliasson, Principles of Yacht Design

Other rules of mixtures Shear modulus: Poisson’s ratio: Thermal expansion: